1. Field of the Invention
This invention relates to adhesive compositions wherein one or more of the reactive constituents are encapsulated. It more particularly relates to encapsulated compositions which upon fracture of the capsules enable commingling of internal and external constituents for reactive contact to form a high strength structural adhesive
2. Description of the Related Art
Douek et al. (U.S. Pat. No. 3,993,815) teaches anaerobic pressure sensitive adhesives formed from reaction of methacrylate esters with diisocyanates.
Douek's resins are combined with a thermoplastic polymer system of one or more high molecular weight polymers which constitutes the adhesive system. Douek provides the anaerobic pressure sensitive adhesive stocks in the form of sheets or tapes from which the adhesive layer can be transferred to a substrate to be bonded.
Azevedo (U.S. Pat. No. 4,417,028) teaches an improved polymerizable binder material compared to the prior art systems of polymerizable acrylate, peroxyinitiator, accelerator and aqueous polyvinyl alcohol binder systems of U.S. Pat. Nos. 3,814,156 and 3,489,599. Azevedo teaches a two part system with encapsulation of the free radical initiator as one part, and the monomer, binder and accelerator as a second part. Azevedo teaches improved binder systems based on replacing polyvinyl alcohols with anhydrides.
Gosiewski (U.S. Pat. No. 5,206,288) teaches an adhesive for low temperature applications involving combining methacrylate ester monomers with elastomeric polymers. The elastomeric compounds form up to 35 weight percent of the adhesive. The elastomeric polymers are selected to have a Tg below −25° C.
Other prior art systems involve solvent based adhesive systems. Roesch et al. (U.S. Pat. No. 5,922,798) teaches a solvent based adhesive system where a water insoluble polymer together with an organic solvent for the polymer is encapsulated and coated onto structures such as sections of plastic piping to join such sections using an interference fit to fracture the capsules releasing the solvent polymer adhesive.
While encapsulated adhesive systems have simplified end-use applications, such systems still have drawbacks limiting adaptation in many end use applications and environments.
With thin layer, dry-to-the-touch pre-applied adhesives, premature failure of the bond, or low bond strength has limited adoption in many industries especially for structural applications.
Degradation of the adhesive during storage prior to use has also been a concern.
Often times, to address concerns with premature failure, the thickness of the applied adhesive coatings is increased resulting in a need to provide latitude in tolerance in specifications to account for volumes and gaps to accommodate the glue loading levels. An adhesive system of high strength that could be pre-applied and activatable after drying would be of commercial interest. It would eliminate the need to handle fluids or wet materials during construction or assembly. Applied as a thin dry-to-the-touch coating would allow tighter product manufacturing tolerances and potentially reduce costs through more efficient utilization of materials and resources. Such an adhesive would facilitate ease of use and more convenient and rapid assembly of parts. A pre-applied dry-to-the-touch adhesive coating of high strength useful as a structural adhesive would be an advance in the art.